1. Field of the Invention
This invention relates to apparatus and methods for digital time multiplexed communication. More particularly, it relates to apparatus and methods for transmitting protective relaying information and includes devices and techniques which enhance the security of such communications by assuring rapid re-synchronization if the signal is interrupted. The invention also concerns such communications in which voice and/or other data are transmitted in the same time multiplexed signal as the relaying information.
2. Background Information
Electric utilities incorporate monitoring and protective relaying equipment into their power systems in order to insure reliable operation and maximum availability. Frequently this equipment must communicate with similar equipment at the ends of long transmission lines. The means of communication is traditionally either microwave or telephone links. Unfortunately, microwave links are susceptible to various environmental conditions such as rain, snow, and fog. Conventional telephone links over either switched or non-switched networks do not meet the reliability and availability requirements for protective relaying. Conventional telephone communication systems such as T1 or T3 networks have not been designed for secure operation and are unable to provide rapid re-synchronization during periods of signal loss.
In a T1 telecommunication system, bytes of data from 24 channels are time multiplexed into a serial bit stream. The bit stream is divided into a sequence of frames, each consisting of one synchronizing bit followed by a byte of data from each channel (193 bits per frame). The synchronizing bits in each group of 12 frames, known as a superframe, are encoded in a unique sequence. The sequence, repeated continuously, is used to synchronize T1 decoders. In an extended frame format, only one bit for four frames is used, and the unique six bit sequence repeats every 24 frames.
Data bytes from any one channel are separated in the T1 format by 23 bytes of data from the other channels. As a result, when the bit stream is demultiplexed at a receiver, synchronization errors can cause channels to be cross-connected, or data from different channels to be intermixed. To prevent the gross effects which could result from such data mixing, the data to all channels is latched and held fixed during periods when synchronization errors are detected at a receiver.
The single bit per frame provided for synchronization in the T1 format is sufficient for maintaining synchronization once it is established, but it does not facilitate rapid re-synchronization when interruption of data reception occurs. In re-synchronizing, there are 193 bit positions in which the lost synchronizing sequence can occur. In searching for the synchronizing sequence, false indications can easily occur since the channel data is unconstrained. Consequently, several repetitions of the sequence at a particular bit position are required before synchronization is assured.
At the T1 data rate, 12 frames are received in 1.5 milliseconds, and three repetitions of the synchronizing sequence therefore require a minimum of 4.5 milliseconds. The actual time required to re-synchronize is a variable which depends on three factors: (1) the number of bit positions that are searched simultaneously (equipment complexity), (2) the number of false indications that happen to occur in the channel data, and (3) the number of positions that must be searched before the correct position happens to be located.
For a protective relay data channel, the maximum re-synchronization time has been specified as two milliseconds. The standard T1 system is therefore unacceptable, and a special synchronizing arrangement is required.
In addition to the above, the communications capability of conventional telephone systems such as T1 , or microwave networks, is severely underutilized when dedicated to a single protective relaying application. There is a need therefore for a communication network which can be utilized to its full capability while providing the security necessary for protective relaying.
Accordingly, it is a primary object to the invention to provide apparatus and a method for secure relaying communication.
A broader object of the invention is provide a method and apparatus for rapid re-synchronization of a time multiplexed communication signal if synchronization is lost.
It is another important object of the invention to provide a method and apparatus which realizes the previous objects while providing full utilization of the communication system capability.
It is a more specific object of the invention to provide a method and apparatus which satisfies the above objects while using a standard T1 format.